A wellhead assembly, such as that employed on the seabed for offshore drilling and production operations, will often comprise an apparatus for retaining equipment and assemblies within the wellhead. An example of such a device is an internal tree cap. Internal tree caps typically comprise a housing for location within the bore of the wellhead assembly. The internal tree cap further comprises an assembly for engaging a suitable means within the wellhead. Suitable means within the bore of the wellhead to be engaged by the internal tree cap include one or more circumferential internal grooves and one or more internal seats or shoulders. Internal tree caps are employed within a wellhead assembly to retain assemblies and equipment. For example, internal tree caps can be employed to retain one or more plugs, such as a wireline plug, to close the central bore of the wellhead assembly. One or more seals are typically disposed between the tree cap and the inner wall of the wellhead assembly. One common location for internal tree caps is within the central bore of a spool tree forming part of the wellhead assembly.
In a further application, an internal tree cap is employed to retain a casing or tubing hanger within the wellhead. Within a well, casings or tubings are installed suspended from a hanger. The hanger is typically landed within the bore of the wellhead assembly against a shoulder. The hanger assembly typically comprises one or more seals to seal against the inner wall of the wellhead assembly. The hanger assembly may comprise one or more assemblies for engaging with the wellhead, for example with one or more circumferential grooves formed in the inner wall of the wellhead assembly.
It has been found that, in use, movement of such devices as internal tree caps and tubing hangers within the bore of the wellhead assembly after installation causes rapid wear of the seals between the respective devices and the inner wall of the wellhead assembly to occur. This in turn leads to a loss in the integrity of the seal.
U.S. Pat. No. 4,408,783 discloses a holddown apparatus for retaining a casing or tubing hanger within a wellhead assembly. The holddown apparatus is mounted on the tubing hanger. The hanger is provided with a shoulder for engaging with a seat in the bore of the wellhead assembly. The holddown assembly comprises a radially expansible locking ring having an upwardly facing tapered surface. An annular cam ring is provided having a corresponding tapered surface to engage with the tapered surface of the locking ring and is threadingly engaged with the hanger. When the hanger and holddown apparatus are located within the wellhead assembly, the locking ring is positioned opposite a circumferential groove in the wall of the wellhead bore. The cam ring is rotated by a running tool and is caused to move against the locking ring moving it into engagement with the inner groove of the wellhead assembly. Removal of the hanger is only possible once the cam ring is rotated in the opposite direction, moving it out of engagement with the locking ring, in turn allowing the locking ring to release from the internal groove of the wellhead assembly. In this way, the holddown apparatus retains the hanger locked within the wellhead assembly. Due to imprecision in the forming of the locking ring and the inner groove of the wellhead assembly, relative movement between the hold-down and hanger apparatus and the wellhead can occur. As discussed above, this leads to a rapid wearing of the seals acting between the hanger assembly and the inner wall of the wellhead.
U.S. Pat. No. 5,307,879 discloses a casing or tubing hanger assembly having a lockdown apparatus for locking the seal of the hanger against the inner wall of wellhead assembly into which the hanger is installed. The lockdown apparatus includes a split lock ring which is movable radially into and out of engagement with a circumferential groove in the wall of the inner bore of the wellhead assembly. A wedge ring is provided for vertical movement. Downwards movement of the wedge ring brings it into contact with the segments of the lock ring, which action moves the lock ring into engagement with the inner groove of the wellhead. Once the lock ring is engaged, the hanger assembly is retained in the wellhead bore. Further downward movement of the wedge ring is provided, which acts to set the seal, once the lock ring is in full engagement with the groove of the wellhead. This further movement is provided by allowing the wedge, ring to partially deform under the action of continued applied downwards force once the lock ring is fully engaged. Again, even though the lock ring of the assembly of U.S. Pat. No. 5,307,879 may fully engage in the internal groove of the wellhead, vertical movement of the lockdown apparatus and the hanger assembly is possible, leading to the rapid wear of the seals discussed above.
It can be seen, therefore, that there is a need for an improved manner of securing and retaining assemblies, such as tubing or casing hangers, within the bore of a wellhead, such that relative vertical movement of the assembly with respect to the wellhead assembly is prevented.
U.S. Pat. No. 5,145,006 discloses a tubing hanger and running tool with preloaded lockdown. The hanger lands on a seat within a subsea wellhead housing. A locking means is provided on the hanger to engage an interior recess in the wellhead housing. A preloading means includes an extendible ring cooperating with the locking means to provide a preload to the hanger and prevent movement of the hanger within the housing.
A tubing hanger has a very specific use within a wellhead assembly, that is the support and retention of tubing extending within the well. A tubing hanger is a complex item of equipment. In keeping with this complexity, it has been found that it can be difficult to provide the tubing hanger itself with the preload means, as well as retain the other features of the hanger necessary for it to function properly. Accordingly, an improved device is required that may be applied within a wellhead, for example to secure a tubing hanger, as well as a range of other apparatus within a wellhead housing, in such a way that relative movement between the device, together with any apparatus it is retaining, and the wellhead housing is prevented.